This invention relates to improvements in surveyors staffs and relates particularly to an improved construction of surveyors staff which allows direct reading of survey levels without the necessity to conduct mathematical calculations.
Plane surveying or level surveying is traditionally carried out utilizing a level, commonly a dumpy level, tilting level or automatic level and a graduated surveying staff or rod. The level is an instrument used to define a horizontal plane, the plane being traced out by the line of collimation of a telescope constrained to rotate about a vertical axis. The traditional surveying staff or rod is generally an elongated rod of wood, metal or fiber glass material having graduations marked thereon which are visible when the rod is viewed through the level telescope. Normally the staff is graduated from zero at the base so that appropriate calculations can be made of the staff readings.
While traditional surveying staffs are formed of a single length of material, several staffs are known which are collapsible either by folding or by a telescopic-type action. One of the most widely used collapsible staffs is the "Philadelphia" rod, which is described in many surveying text books such as Elementary Surveying (5th edition) published by International Text Book Company, Scranton, Pa. (published 1970).
Australian Pat. No. 272691 issued to Mansfield J. Mitchley, dated Oct. 24, 1973, discloses another form of levelling staff in which one face of the staff has the traditional set of graduations and a second set of graduations are provided on a single length of movable tape supported on the other face of the staff. The movable tape is supported against the surface of the staff and is capable of sliding movement in a direction substantially parallel to the axis of the staff, the graduations of the movable tape being arranged to increase downwardly from the top. The tape is adapted to be wound around reels at each end of the staff. A similar construction of levelling rod is disclosed in Australian Pat. No. 417276 issued to Fredric H. Schneider, dated Nov. 2, 1967. This specification discloses an elongated levelling rod having a movable template strip extending along the rod and being furled at opposite ends thereof in coils. A plurality of measurement marker elements are removably secured to the face of the movable strip at predetermined positions. An index point is carried on a first of the furling means, and a target marker element is carried on the face of the strip at a predetermined position.
These proposed constructions of surveying staffs suffer the disadvantage that when the movable strip reaches the end of its travel the usefulness of the staff is substantially restricted.
A further known construction of levelling staff utilizes an endless band or tape which passes over rollers at each end of a solid timber rod. The tape or band is of metal and is of approximately ten (10) feet in length. With this construction, the range of levels which can be read from one setting of the tape is very limited. Further, the tape can move between readings thus substantially reducing accuracy of subsequent readings.